Customer portal Invite email

Hi, we have a customer portal in which some user are working well but we need to add more users and the issue is that the invite email is not sent. We have sent the re-invite but the user does not receive the email

Is there any way to see what the problem is and how to solve it?

I think that there is no limit for customer portal users in free license but I would like to confirm it.

Many thanks.

Regards

Samuel